The present invention relates to a unit for sterilizing strip material on a packaging machine for packaging pourable food products, and to a packaging machine comprising such a unit.
Machines for packaging pourable food products, such as fruit juice, wine, tomato sauce, pasteurized or long-storage (UHT) milk, etc., are known in which packages are formed from a continuous tube of packaging material defined by a longitudinally sealed strip.
The packaging material has a multilayer structure comprising a layer of paper material covered on both sides with layers of heat-seal material, e.g. polyethylene. In the case of aseptic packages for long-storage products such as UHT milk, the packaging material comprises a layer of barrier material defined, for example, by an aluminium film, which is superimposed on a layer of heat-seal plastic material and is in turn covered with another layer of heat-seal plastic material which eventually defines the inner face of the package contacting the food product.
For producing aseptic packages, the strip of packaging material is unwound off a reel and fed through a sterilizing unit in which it is sterilized, for example, by immersion in a bath of liquid sterilizing agent such as a concentrated solution of hydrogen peroxide and water.
More specifically, the sterilizing unit comprises a bath filled, in use, with the sterilizing agent in which the strip is fed continuously. The bath conveniently comprises two parallel vertical branches connected at the bottom to define a U-shaped path of a length depending on the traveling speed of the strip and such as to allow enough time to treat the packaging material. For effective, fairly fast treatment, so as to reduce the size of the sterilizing chamber, the sterilizing agent must be maintained at a high temperature of, say, roughly 70xc2x0 C.
The sterilizing unit also comprises an aseptic chamber in which the strip of packaging material issuing from the sterilizing bath is treated to remove any residual sterilizing agent. The amount of residual sterilizing agent allowed in the packaged product, in fact, is governed by strict standards (the maximum permissible amount being in the order of a few parts per million).
The above treatment normally comprises a preliminary operation to mechanically remove any drops on the material, and which is normally performed by means of two cylindrical drying rollers conveniently located close to the input of the aseptic chamber, and between which the material is fed., Downstream from the rollers, the material is still wet but with no macroscopic drops, and is fed between two air knives by which any residual traces of sterilizing agent are evaporated.
Before leaving the aseptic chamber, the strip is folded into a cylinder and sealed longitudinally to form in known manner a continuous, vertical, longitudinally sealed tube. The tube of packaging material, in fact, forms an extension of the aseptic chamber and is filled continuously with the pourable product and then fed to a forming and (transverse) sealing unit for forming individual packages and by which the tube is gripped between pairs of jaws to seal the tube transversely and form aseptic pillow packs.
The pillow packs are separated by cutting the sealed portions between the packs, and are then fed to a final folding station where they are folded mechanically into the finished form.
Packaging machines of the above type are used widely and satisfactorily in a wide range of food industries to form aseptic packages from continuous flat strip material; and performance of the sterilizing unit, in particular, is such as to amply conform with standards governing asepticity of the packages and residual sterilizing agent.
Within the industry, however, demand for further improvement exists, especially as regards elimination of residual sterilizing agent, and which stems, in particular, from market demand for packages featuring reclosable opening devices which are easy to open and provide for easy pouring of the product.
In the case of nonaseptic packaging machines, such devices are applied, e.g. injection molded directly, to the strip material before the packages are formed.
Conversely, in the case of aseptic packaging machines, any opening devices are normally applied after the packages are formed. That is, if applied beforehand to the strip, the opening devices form breaks in the geometric continuity of the strip, in which residual sterilizing agent may become trapped, and from which the sterilizing agent cannot be removed completely using known techniques. In particular, the opening devices project from the packaging material and prevent use of the aforementioned known drying rollers.
Applying the opening devices to the finished packages poses drawbacks from the production standpoint by requiring complex, high-cost systems for supplying and applying the devices.
It is an object of the present invention to provide a unit for sterilizing strip material, in particular on a machine for packaging pourable food products, designed to solve the aforementioned problems, and which in particular provides for forming aseptic packages from strip packaging material to which opening devices are applied beforehand.
According to the present invention, there is provided a unit for sterilizing strip packaging material on a packaging machine for packaging pourable food products, the unit comprising a bath for containing a sterilizing agent in which the packaging material is fed continuously; and an aseptic chamber having an input connected to an output of said bath and housing two drying rollers cooperating with opposite faces of said packaging material; characterized in that said rollers have respective recesses at a longitudinal portion of said packaging material having a succession of preapplied opening devices; said unit comprising auxiliary means for removing residual sterilizing agent from said longitudinal portion of said packaging material.
By virtue of the recesses on the drying rollers, the drying rollers may also be used on packaging material with preapplied opening devices; and the combined action of the drying rollers and the auxiliary means for sterilizing the portion of the material to which the opening devices are applied, and which is unaffected by the drying rollers, provides for removing all the sterilizing agent from the material.
In a preferred embodiment, the auxiliary means are defined by two nozzles supplied with sterile air.
The sterile air supplied to the nozzles is preferably drawn from the aseptic chamber by means of a recirculating circuit.
The present invention also relates to a packaging machine for producing packages of pourable food products from strip packaging material, the machine comprising a sterilizing unit for sterilizing said packaging material and in turn comprising a bath for containing a sterilizing agent in which the packaging material is fed continuously, and an aseptic chamber having an input connected to an output of said bath and housing two drying rollers cooperating with opposite faces of said packaging material; and applying means for applying opening devices for said packages; characterized in that said applying means are located upstream from said sterilizing unit to apply said opening devices successively to a longitudinal portion of said packaging material; and in that said rollers have respective recesses at said longitudinal portion of said packaging material; said sterilizing unit comprising auxiliary means for removing residual sterilizing agent from said longitudinal portion of. said packaging material.